It sounds as if you have a heavier guage Pipers Plaid Brooch rather than the lighter brooch suitable for fly plaids

If so you could simple pin the brooch to the plaid and use a safety pin to anchor the plaid to the jacket. If the fly plaid is light enough you may even be able to get away without pinning it to the jacket

A couple of comments though. A fly plaid is not usually recommended for wear with a PC and is more appropriately worn with a doublet. Second, a smaller brooch looks far better with a fly plaid and is not expensive to purchase